Epoxy compositions and their curing techniques are well known and the patents issued on curable epoxy compositions number in the hundreds. Known curing agents include, among others, polycarboxylic acids and anhydrides, amines, polyamides, imidizoles, and the like. Representative curing agents are described in U.S. Pat. No. 3,336,241. These curing agents may be employed with one or more catalysts or accelerators such as the stannous salts of monocarboxylic acids.
It will be appreciated that each and every one of the known epoxy-curing systems exhibits advantages over other systems, and, as importantly, disadvantages over the same systems. There is, of course, a continuing need to develop better epoxy curing compositions.
The fabrication of thermoset polyester resin articles from curable compositions comprising an ethylenically unsaturated polyester dissolved in a liquid ethylenically unsaturated monomeric crosslinking agent which is capable of polymerizing with the polyester is well known. Articles having relatively high strength and low density can be produced from such compositions by incorporating therein fibrous reinforcements such as glass fibers. Fillers, such as calcium carbonate and clays, are usually added to such compositions as extenders. See for example, U.S. Pat. No. 3,959,209.
For convenience, the term "polyester resin" when used herein refers to a composition which contains as an essential ingredient an ethylenically unsaturated polyester and an ethylenically unsaturated reactive diluent and which may contain other ingredients such as fillers, fibers, reinforcements, curing catalysts, etc.
Polyester resins are widely used in molding applications in liquid form. Such liquid resins comprise a liquid solution of a liquid or solid polyester dissolved in a liquid crosslinking agent, for example, styrene monomer, the most widely used reactive diluent.
For many types of molding applications, it may be desirable for the polyester resin to be in solid form, for example in the form in sheets, granules or powders. Solid forms of polyester resins can be made from the liquid polyester resin solutions mentioned above. For example, a normally liquid polyester resin solution can be converted into solid form by the addition thereto of a chemical thickening agent such as an oxide or hydroxide of magnesium or calcium. Or the liquid polyester resin solution can be converted into solid form by adding thereto a solid filler, such as calcium carbonate, which absorbs the liquid resin. Liquid or solid amorphous polyesters which are soluble in the liquid crosslinking agent are conventionally used in this type of application.
There is a need for a curable thermosetting resin composition which has improved tensile strength, tensile modulus, and glass transition temperatures while adequate tensile strain to break is maintained, over that of "pure" epoxy resin or "pure" polyester resin.